The use of mechanical insufflation and exsufflation (MI-E) with negative pressure is a well-known technique for helping patients with an ineffective cough to remove secretions from the respiratory tract. Patients who can benefit from the technique include: post-polio, muscular dystrophy, spinal muscular atrophy (SMA), post-cardiac surgery, amyotropic lateral sclerosis (ALS), mechanically ventilated, or anyone with insufficient muscle strength to generate the high expiratory flows necessary for moving secretions up the tracheo-bronchial tree. The technique involves the use of a blower and valve, which via a facemask, mouthpiece or adapter for a tracheal tube, alternately applies positive pressure first to inflate the lungs, then shifts rapidly to negative pressure to create a high expiratory flow. One shortcoming of MI-E is its limited effectiveness on patients with unusually thick or tenacious secretions. In such patients MI-E can create high expiratory airflows, but the secretions tend to remain attached to the airways within the lung and cannot be dislodged or loosened.
The use of percussion in loosening secretions in patients' airways is also a well-known technique. The most widely used is referred to as “chest physical therapy”, wherein a nurse or physical therapist uses cupped hands to rhythmically strike the chest wall, producing vibrations in the lung tissue. There are also devices that accomplish this percussion both external to the body via mechanical “thumpers” or positive pressure vests, and internally via a facemask or mouthpiece, using short bursts of positive pressure to the airway. In addition, some devices, like the “Flutter®” (Axcan Scandipharm) and the “Acapella®V” (DHD Healthcare) use the patient's own expiratory flow to generate oscillations in the airway.
A shortcoming of these various percussive techniques is that they effectively loosen secretions in a patient's airways, but do not actively remove the secretions, as does MI-E. In fact, for a patient with insufficient peak cough expiratory flow, the use of percussion alone may loosen a mucus plug, which the patient cannot expel, causing blockage of the airway. Also, patients with neuromuscular disease or general muscle weakness cannot generate adequate airflow to use the “Flutter®” or “Acapella®” devices.
One approach, U.S. Pat. No. 2,918,917, applies a vibration to either a positive pressure, (inhale), or negative pressure, (exhale) but not both. This has not been satisfactory in some applications. Also this approach employs a diaphragm pump to fluctuate the air provided to the patient. Such pumps are large and not conducive to portability and require significant energy in addition to that used to drive the primary pressure source. In addition, in applying pressure oscillations this approach must work against the pressures provided by the primary pressure source itself.